1. Field of the Invention
The present invention relates to a mask for crystallizing silicon and an apparatus having the mask. More particularly, the present invention relates to a mask for crystallizing silicon capable of improving electric characteristics of silicon and an apparatus having the mask.
2. Description of the Related Art
In general, an amorphous silicon thin film transistor (a-Si TFT) has been used as a switching element in a liquid crystal display apparatus. Recently, a poly crystalline silicon thin film transistor (poly-Si TFT) having a higher operation speed has been used for the liquid crystal display apparatus to display an image of a high display quality. Particularly, in an organic light emitting display (OLED) device having an organic light emitting diode that is driven by a current, the poly-Si TFT has been widely used for a switching element, a driving element, etc.
The method of forming a poly crystalline silicon thin film of the poly-Si TFT has included directly forming the poly-Si TFT on a substrate, and forming an amorphous silicon thin film on the substrate and then heating the amorphous silicon thin film to form the poly crystalline silicon thin film, etc. A laser has been generally used for the heating step.
According to the heating method using the laser, a laser beam generated from the laser melts the amorphous silicon thin film on the substrate. The melted silicon is crystallized into a plurality of grains that grows around a plurality of nuclei to form the poly silicon thin film having good crystalline characteristics. Therefore, the amorphous silicon thin film is changed to the poly crystalline silicon thin film that has a higher electric conductivity than the amorphous silicon thin film.
The laser beam generated from the laser may be directly irradiated onto the substrate in one example, or it may be irradiated through a mask in another example. The mask includes a plurality of slits for transmitting the laser beam.
The mask having a small size is transported on the substrate in horizontal and longitudinal directions of the substrate to irradiate the laser beam to an entire surface of the substrate. That is, the mask is transported on the substrate in the longitudinal and horizontal directions at a predetermined distance, and the laser beam is then irradiated onto the substrate so that substantially the entire amorphous silicon thin film of the substrate is changed to a poly crystalline silicon thin film.
However, when the mask is transported to the longitudinal and horizontal directions, a scanning of the laser beam is overlapped with adjacent scanning of the laser beam. That is, a portion of the silicon thin film on the substrate is repeatedly exposed to the laser beam on the substrate. The substrate is divided into a first part where the laser beam is irradiated once, and a second part where the laser beam is repeatedly irradiated. The first part and the second part have different electronic characteristics.
Moreover, when dividing the first part and the second part according to the amount of the irradiation of the laser beam, a structure of the poly silicon thin film on or near a boundary between the first and second parts on the substrate is different from that of a remaining portion of the poly silicon thin film. The boundary deteriorates the electrical characteristics of the poly crystalline silicon thin film, such as electric conductivity.